The present invention relates to a fiber-optic communication system.
Next-generation high-capacity fiber optic communication systems now at the planning stage are proceeding toward a practical realization through utilization of conventional, time-tested non- return-to-zero (NZR) lightwave pulses and a novel optical amplifying-repeating technique which is an alternative to the traditional regenerative repeater technique, and it is expected that commercial fiber optic communication systems having a transmission capacity of 5 Gbit/s to 10 Gbit/s will go on stream within two or three years. In a future ultra-large-capacity optical communication system which will succeed them, the required bit rate becomes as high as tens of gigabits per second or above and the corresponding pulse width of the lightwave signal becomes as small as tens of picoseconds or below; hence, even a slight increase in the pulse width during transmission would result in serious degradation of transmission quality. For this reason, attention is now being focused on optical transmission technology which uses soliton lightwave pulses (return-to-zero (RZ) lightwave pulses of substantially a sech.sup.2 type pulse waveform) whose optical wave form is said to hardly undergo degradation even by long-distance transmission, and its research and development is being carried out with the objective of providing a primary optical communication system which supports a future highly information-oriented society.
Usually, when lightwave pulses are transmitted over an optical fiber, their pulse width broadens by the wavelength dispersion characteristic of the optical fiber owing to frequency spreading inherent in data-modulated lightwave pulses. The freedom of the soliton lightwave signal from variation in its pulse waveform by transmission is achieved when the pulse width compression, which is caused by frequency chirping of the lightwave pulses owing to the optical nonlinealities of the optical fiber forming the transmission line and the wavelength dispersion characteristic of the transmission line, balances with the afore-mentioned pulse width broadening. Accordingly, to accomplish the optical soliton transmission which maintains theabove-mentioned balance and hence is almost free from variations in the lightwave pulse waveform, it is necessary that the zero dispersion wavelength of the transmission line be shorter than the wavelength of the lightwave signal to hold a desired wavelength dispersion characteristic (see "Soliton Propagation in Long Fibers with Periodically Compensated Loss," L. F. Mollenauer, J. P. Gordon, and M. N. Islam, IEEE Quantum Electron., QE-22, pp.157-176, 1986).
In the optical soliton transmission which is almost free from waveform degradation by transmission, timing jitter which is brought about by various causes during transmission constitutes a main factor that determines the transmission characteristic, along with degradation of the SN ratio by accumulated optical noises. The Gordon-Haus jitter, which forms a main part of such timing jitter, is attributable to such a phenomenon as described below. That is to say, in the optical amplifying transmission system the optical soliton carrier frequency, which randomly fluctuates due to optical noises produced by optical repeater-amplifiers, is converted mainly by the wavelength dispersion characteristic of the fiber optic transmission line into fluctuations in the system propagation time (see "Random Walk of Coherently Amplified Solitons in Optical Fiber", J. P. Gordon and H. A. Haus, Opt. Lett., vol. 11, pp.665-667, 1986). The Gordon-Haus jitter increases with distance, and hence exerts a great influence onlong-distance soliton transmission; furthermore, the pulse spacing reduced by the Gordon-Haus jitter increases the interaction between adjacent optical soliton pulses, newly causing timing jitter. Thus, the Gordon-Haus jitter is an important problem yet to solve for putting the system to practical use.
It is effective in suppressing the Gordon-Haus jitter to make the optical nonlinear constant and wavelength dispersion of the transmission line small, the mode field diameter of the optical fiber large, the pulse width large and the amount of optical noise small, but it is known that the jitter cannot easily be reduced for such reasons as listed below. 1) An increase in the pulse width causes a decrease in the pulse spacing, resulting in the generation of timing jitter by the interaction between adjacent pulses. 2) Too much reduction of the wavelength dispersion decreases the power of the corresponding lightwave pulse by the afore-mentioned balancing mechanism, providing increased degradation of the SN ratio by accumulated optical noise. 3) The optical nonlinear constant of the quartz optical fiber is small by nature and its substantial reduction cannot be expected. 4) It is not desirable to set the mode field diameter of the optical fiber to a value larger than that at present, because it will deteriorate the bending loss characteristic. 5) The noise figure of the optical amplifier at present is already appreciably close to its theoretical limit and the reduction of the noise figure that can be expected in the future is as small as 1 dB or so.
For the reasons given above, it is customary in the prior art to insert optical elements for jitter reduction use in the transmission line of an ordinary soliton transmission system, by which the timing jitter could be suppressed to some extent. At present, however, the jitter suppressing mechanism is too complicated to be practical and the timing jitter is not suppressed much enough to provide a system margin satisfactory for practical use.